Pipe-jacking helps preserve Welsh SSSI

Construction of an interceptor sewer beneath a protected marsh has been completed by Byzak Contractors, as part of a £180M Welsh Water scheme to improve Cardiff¹s sewerage facilities.

Construction of Welsh Water's £36M Cardiff east interceptor sewer has been successfully completed by Byzak Contractors. The 10km sewer will link four sewers, which currently empty directly into the Bristol channel, with a new £180M STW to the south of the city. The majority of the sewer was constructed using an advanced pipe-jacking technique.

Pipe-jacking can be less expensive than segmental tunnelling, does not involve open-cut trenching and can be used to lay pipelines up to 3m in diameter.

In the Cardiff project, the 4km furthest east has been jacked with 1,650mm pipes, and the central section with 2,000mm pipes. The only section to be segmentally tunnelled was the 2,400mm, 2.5km section closest to the STW, which had to complete a curved path under the estuary of the River Rhymney. All of the pipes used were concrete; the upstream section from Hanson (ARC), the central section from Hepworth, and the downstream segments from Charcon. Pipe-jacking is carried out as follows; firstly, vertical shafts are sunk at intervals along the route. Once the ground has been stabilised, for instance by concreting to protect against groundwater inrush, an Œeye is made in the side of the shaft for the tunnelling machine to enter at the required depth. A thrust jacking wall is also built into the base of the first shaft, to withstand the force of jacking (up to 600t per jack).

The tunnelling machine is then lowered into the shaft. Once tunnelling has begun, the first pipe is lowered into position behind the machine, followed by the jacks. A series of pipes can then be jacked into the tunnel as the machine proceeds, with the aid of a lubricant. Rock/slurry from the tunnel face is transported back out through the pipes.

The longest single drive on the Cardiff project was 520m, using Herrenknecht's laser-guided, remotely-controlled tunnelling machines. Connections from the four outfalls to the interceptor sewer will be made later next year, when the Cardiff city STW will finally come on-line, serving a pe of nearly 0.9M.


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